1. Field of the Invention
The present disclosure relates to semiconductor devices, and more particularly, to methods for controlling sleep modes of semiconductor devices.
2. Description of the Related Art
Some semiconductor devices include a sleep mode function. In order to reduce power consumption of a semiconductor device, a control system may apply a control signal to a semiconductor chip that is expected not to operate for a period of time, thus placing the semiconductor device into a sleep mode. A sleep mode is also sometimes referred to as a standby mode or state, a stop mode, or a power down mode. In each mode, the level of source voltage applied to the semiconductor device is reduced. When a semiconductor device is used in a portable device, a sleep mode function of the semiconductor device has a great effect on reducing power consumption of the portable device. An example of a conventional semiconductor device having a sleep mode function is disclosed in U.S. Pat. No. 6,137,744.
On the other hand, a semiconductor device in a sleep mode can be placed back to an active mode when required. When switching from a sleep mode into an active mode, a semiconductor device has to return to a state prior to the switch to sleep mode, and operate normally. Generally, when a semiconductor device operates in a normal mode, data generated by internal combinational circuits is stored in a temporary memory space such as internal flip-flops. However, since power sources supplied to most of the circuits in the semiconductor device are cut when the semiconductor device is switched to the sleep mode, the data stored in the flip-flops is lost. Accordingly, when such a semiconductor device is switched again from a sleep mode to an active mode, the semiconductor device cannot recover its previous state and does not operate properly.
The problem described above can be solved if the flip-flops are not placed in a sleep mode even if the semiconductor device is placed in the sleep mode. However, a solution as described above may restrict any reduction of power consumption when the semiconductor device is in the standby state. In order to prevent data loss and reduce power consumption of the semiconductor device in a standby state when entering a sleep mode, the semiconductor device can store data in an additional storage space and be switched to a sleep mode. However, in this case, since the semiconductor device has to include additional circuits for storing the data, such as circuits forming a path in which the data is transferred to a storage space such as a memory, the manufacturing cost of the semiconductor device is increased.